Pl. 11, 12.

The skull of Dimorphodon differs in form and in many important details of structure from that of Rhamphorhynchus; and both of these types of skull are strikingly unlike that of the short-tailed animals named Pterodactyle. Hence, as it will be shown that the Cretaceous fossils of this class belong to very distinct new genera, there is no reason for assigning to them by anticipation any class of cranial structures. The cranium of this type of animal has never been critically described, and for all that is yet known to the contrary Pterodactyles may differ between themselves as much as birds or mammals. Their affinities have been unknown. Therefore, before describing bones it may be desirable to state the grounds on which the several specimens are referred to the Ornithosauria. The fossils on which this section of the memoir is founded are, the basi-occipital and basi-temporal bones, the anterior portion of a cranium, the back parts of four crania, facial bones, and the quadrate and quadrato-jugal.

The crania are all no larger than that of the Heron; though from the Greensand are bones and jaws indicating Pterodactyles both smaller and larger. The skulls are mostly remarkable for wanting both basi-occipital and basi-temporal bones. And the specimen of basi-temporal and basi-occipital corresponds posteriorly with the Pterodactyle atlas, anteriorly with these crania; it is hence concluded to have belonged to a similar animal. Being relatively twice as large, it indicates that in these animals the basi-occipital condyle was proportionally larger than in known birds; and that animals of a cognate kind had skulls probably twice the size of these. The anterior basal part of the hinder sphenoid terminates in a remarkable triangular surface, with two perforations, which are separated by a median ridge. Almost entirely corresponding with this is the basal surface of the anterior part of a cranium, fractured in front of the pituitary fossa. Therefore, and as it indicates a similar capacity of brain, it is regarded as belonging to the same kind of animal as the others ; but being five times the size, it must, if the proportions of the Heron were preserved, have been part of a head a yard long.

Now, as there is no other animal with the same texture of bone, or exhibiting with high organization the same diversity of size, these cranial fragments are referred to the jaws and bones of Pterodactyle. So marked are their structures that many quarry-men refer vertebrate fossils to their several orders with almost as much accuracy as would a practised anatomist.

Basi-occipital, Owen, Sup. Cret. Rep. p. 6, T. 1, figs. 11, 12, 13.

This bone was not found associated with any set of fossils that would induce us to refer it to one species more than to another. Its Ornithosaurian character was probable; and Prof. Owen described it in his last memoir on the Greensand Pterodactyles.

But though indubitably basi-occipital, it is so anomalous in some respects that the Professor regarded the under as the upper surface; since then the investing phosphate of lime has been removed, and the bone is now described in what appears to be its natural position.

Viewed from above the fossil divides into two parts; the occipital condyle, and an anterior, wide, transversely oblong extension terminating at each side in a strong short horn. The posterior half of the condyle shows large cancelli as though so much of it had been covered by the articular cartilage. The sides of the condyle converge, so that posteriorly it is only two-thirds of the width it has at the foramen magnum, which would appear to indicate a comparatively slight lateral motion of the head. The condyle is hemispherical posteriorly and superiorly; there is a depression between it and the great foramen of the skull; inferiorly it is flat.

It is ⁷/₁₆ of an inch long; posteriorly ⁹/₁₆ wide, nearly ⁶/₁₆ of an inch high anteriorly. It terminates in front superiorly in an elevated transverse ridge.

On removing the matrix, the anterior surface of this occipital bone was found to be concave; yet as nothing but cancellous structure is seen it may be but imperfectly ossified or more probably, imperfectly preserved. And the bottom of this cup expands forward in a thin sheet of bone a quarter of an inch long and half an inch wide, which on the under side is continuous with the base of the condyle.

On each side of this floor and partly extending in front of it, and below it, is an irregular piece of bone, half an inch long, resembling anterior zygapophyses of cervical vertebrÆ.

Though in most vertebrates the basi-occipital enters into the basal floor of the skull, the median bones are either so placed that they rest one upon another from before backwards or abut against one another nearly perpendicular, so that the basi-sphenoid comes commonly to underlap and partly hide the basi-occipital. Nowhere among Amphibia or Reptilia do I know of the reverse position occurring. In some fishes there is an approach to it. Thus a slight anterior bony expansion of the basi-occipital in the Cod fits partly into a horizontal slit in the basi-sphenoid[A]. In the Carp the basi-occipital has a spathulate basal expansion like that of Pterodactyle, but it is underlapped by the basi-sphenoid[S]. In some mammals the under side of the basi-occipital extends further forward than does the neural side, as for example in the Sheep and Goat; while in a few others, as in the Walrus, the reverse positions obtain.

But it is among Birds that the structure described in Pterodactyle is evident and characteristic. For although the bony plate under the sphenoid,—Mr Parker's basi-temporals,—is mostly anchylosed to the bones about it, and less with the occipital than with others, its position and relations are quite the same as those of the expanded flap of this Pterodactyle basi-occipital. Therefore it is identified with the basi-temporal bones.

This fossil is an inch high, rather wider, and half an inch long. It well shows the bones at the back of the skull, the basi-cranial bones, and the bones posterior to the frontals, which roof in the Cranium. There are in it striking resemblances to the back of the skull of some Natatores, as the Grannet and Cormorant, and of some Grallatores as the Heron, and Gallinaceous birds as the Cock.

The base of the skull. The bones here indicated are the basi-occipital, basi-temporal, and basi-sphenoid. The former two have come away as from an articular joint, and are wanting. The basi-occipital does not enter into the floor of the cranial cavity, and only rims the foramen magnum. But its basi-temporal expansion rests beneath the posterior part of the basi-sphenoid forming the base of the skull; its long convex anterior end fits into the concave groove at the back of the anterior part of the sphenoid. The squamous basi-temporal bone appears in this species to have been as long as the foramen magnum is wide, and to have been relatively thicker than in the other form already described.

The basi-sphenoid is a thin expanded bone forming the floor for the cerebellum, and terminating anteriorly in a triangular mass, while the slightly convex part behind, covered with the basi-temporals, is nearly square. It enters into the foramen magnum, forming its lower part; and is confluent with the ex-occipitals behind, with the periotic, alisphenoid and perhaps with the squamosal at the side; and as in birds all these sutures are obliterated. This is probably the only instance in the Animal Kingdom in which the basi-sphenoid takes so important and singular a share in the functions of the basi-occipital bone. The anterior part of the basi-sphenoid projects below the posterior part, is nearly flat on the basal surface, and forms an equilateral triangle with the apex in front and base behind. In the middle of the triangular bone is a slight longitudinal ridge, and behind the middle of each outer side a rather large foramen which appears to be the inferior opening for the carotid artery. The triangular part is hollow and as long as the quadrate portion. The lateral parts of this anterior bone are nearly flat. They converge upwards and are rounded in front to form the boundary of the pituitary fossa, and do not appear to have terminated in a spine. Above are the alisphenoids.

The upper part of the skull is divided into two segments by a strong straight transverse ridge, which leaves the occipital bones behind, and the parietal &c. in front.

The occipital bones anchylosed together are about two-thirds the width of the foramen magnum, and of the parietal bones, with which latter the supra-occipital makes an angle of 45°. The surface is irregular, and especially is marked by a deep concavity just above each ex-occipital. The supra-occipital projects slightly over the plane of the foramen magnum, to which the strong ridge bounding the segment in front is parallel. The great foramen is nearly round, being slightly compressed at the upper part of the sides: it measures ³/₈ of an inch high and is nearly as wide.

The occipital bones make with those at the base of the skull an angle of about 145° or 150°. In outline they are a transverse diamond shape. The mastoid portion is not to be distinguished from the other bones, but appear to terminate the sides of the strong occipital crest, which by posterior compression of the squamosals and parietals, becomes very strong, and makes the backward boundary of the temporal foss. This crest is in the same plane with the anterior border of the basi-temporals.

The parietals meet above in a slight ridge. They are two rectangular bones twice as wide as long, forming a semicircular roof for the brain, which looks outward and a little backward. Anteriorly these bones unite with the frontals in a slightly flexuous transverse line; and inferiorly they are connected with the periotic, the squamosal, and perhaps with the anterior point of the alisphenoid: they do not descend to the plane of the articulations of the free quadrate bones. The surface is smooth, and on the upper part flat, but concave below from side to side.

Below these parietals are the squamosals and alisphenoids, but the suture between them is not seen. They are in form a trapezium where the short side is anterior, and the lower third is folded inward so as to be confluent with the anterior part of the sphenoid. The fold forms a ridge, which I suppose may run obliquely over the alisphenoid. The unfolded squamosal part is a flat and smooth oblong, with parallel sides, the bones are in parallel planes and nearly perpendicular to the base of the skull. Where the alisphenoid joins the sphenoid, there is a considerable concavity, above which is a small circular impression. These strips approximate inferiorly, so that the width of the skull there is rather more than half what it is at their outer margins. They shut off the pituitary body in front of them, and appear to form part of the wall for the orbit of the eye.—The slightly convex, lateral, squamosal parts above the fold continue the circular transverse outline of which the parietals are the upper half. They extend anterior to the parietals, and on the inside give attachment to the frontals. Like the parietals, they make a sharp bend outward at their hinder border, and form the lateral terminations of the occipital ridge, which is the widest part of this fossil.

The only portion of the specimen now to be described is the large region at each side looking downward, which extends from the occipital ridge to the sphenoid. It is an irregular pentangular hollow with many cavities, the hinder of which are for the ear. Two cavities above these, under the widest part of the skull, appear to be a double articulation for the quadrate bone. The outer transverse one with the squamosal is separated by a deep groove from the inner and more vertical one, which may therefore be regarded as with the petrosal bone. These excavations form the posterior half of the pentagon. The anterior half is a smooth rhombus not separable from the basi-sphenoid.

Such is the external appearance of the occipital and parietal segments of the skull of a Cambridge Pterodactyle. Each segment forms a large ring of thin bone, inclosing part of a brain-cavity as large as that of a bird and shaped like that of a bird; and which moreover is made up of the same bones as the cranium of a bird; and these are in almost exactly the same proportions as those of the Common Cock.

My own investigations do not substantiate Wagner's discovery, that the back part of the skull resembles that of the Monitor. Iguana would have offered a slightly nearer comparison, but they both differ from Cambridge specimens of Pterodactyles in characters like these.

In the lizard,

While the peculiar backward development of wings of the parietal in a diverging V form, give the Lizard skull an aspect of its own.

So that it must be asserted that the differences of these Pterodactyles from Lizards are so wide as to preclude comparison.

With the Crocodile, in which the cranial bones are massive, and the quadrate bone firmly packed in the skull, comparison would be no less difficult.

The DelphinidÆ, in both the form of the jaws and of the back of the head, give some support to Wagler's fancy, in putting the Pterodactyle into his curious creation, the Gryphi[T]. But in the porpoises the parietal bones form as narrow a band as they do in the Duck; and are quite unlike the bones here described. In the Dolphin the two condyles almost unite into one semicircular condyle (in young specimens), owing to the enormous development of the ex-occipitals, which almost if not entirely exclude the basi-occipital from the foramen magnum. The dolphin moreover has no quadrate bone. But notwithstanding the absence of a division into occipital and parietal segments, the form and arrangement of the bones in the skull of the porpoises approximate more to the Cambridge Pterodactyles than is the case with Lizards.

But with Birds the correspondence is so close that it would be difficult to discover differences. That one of the condition of the occipital bone seems to be the most important; another is, that from the relatively smaller size of the cerebellum the parietal bones appear to cover a larger part of the cerebrum; and a third is the strong triangular condition of the sphenoid in front of the sella tursica. With these exceptions there is nothing to distinguish the fossil described from the cranium of a bird.

Another cranium has occurred which must be referred to a different genus. Its preservation is less perfect, but it similarly exhibits the occipital and parietal segments of the skull. All the bones are blended together without a trace of a suture.

The occipital region is flat. Its outline is not defined owing to the extent to which the sharp crest, in which it terminated outwardly, has been broken away. The occipital condyle is broken off. The foramen magnum is of an ovate form—flattened at the base. The ex-occipitals at its sides are impressed as though from contact with the neurapophyses of the atlas. Mesially, over the foramen magnum is a vertical elevated crest (now rubbed away), which may have given attachment to a bone like that post-superoccipital crest described by Quenstedt in the Pterodactylus suevicus. The occipital region makes a great angle with the flat basi-temporal region, as in birds.

The parietal region is convex from below upward, the lateral parts converging towards the crown, which however presents a broken and worn surface. From side to side the squamosal and parietal bones are concave, owing to the extended occipital crest behind, and the rapid widening of the skull in front caused by the large size of the brain.

In front is seen a section of the brain-cavity. It is very like in form to the two halves of a pear put together side by side with the stalk downward. I have removed some of the phosphate of lime from the brain-cavity, and although it has not been excavated to the cerebellum, the great depth of the brain is well seen, and the convex character of the cerebral lobes, between which a crest of bone descends mesially as in the ethmo-sphenoid mass next described. At each of the lower outer angles of the brain, extending into the cancellous brain-walls to the outermost film, is an ovoid convexity, covered with a thin film of bone. They entirely correspond with the optic lobes, being in exactly the same position as in birds, only relatively rather small. Underneath the optic lobe on the outside is a small concavity, apparently the articulation for the quadrate bone. The basi-sphenoid mass below the brain is of considerable height, the upper half flat and smooth, the lower half fractured and cancellous.

In the main this skull is like the other one, differing chiefly in the depth of the sphenoid, in the mesial ridge between the cerebral lobes, in showing the optic lobes, and in having anchylosed basi-temporal bones. There would hence appear to have been considerable variations in the skulls of Pterodactyles even in the Cambridge Greensand.

The symmetrical bone which I have so named is a wedge-like mass tapering in front, keeled above; flattened below, and cupped behind on each side. It belonged to a very much larger animal than the last fossil, and probably to a very different genus.

The inferior surface is triangular, an inch and an eighth wide behind, at the base, and an inch and a quarter long; but it is broken at both ends. In its longitudinal median line is a strong keel stopping short in front, dying away behind, and forming with the compressed margins a considerable hollow on each side, at the back part of which is a large oval foramen. This surface, though five times the size, corresponds in form, ridges, and foramina with the anterior part of the sphenoid described in the article on the back of the cranium.

The posterior surface is at right angles to the inferior one, but its lower third shows only fractured phosphate of lime filling perhaps the anterior part of the pituitary fossa. Its upper part also is broken. But on each side is a large concavity measuring in the fractured fossil an inch and a quarter high, three quarters of an inch wide, and half an inch deep from the unbroken median ridge where the cups become confluent at their base. The whole specimen is two and a quarter inches high. From the determination of the under side it follows that these smooth hollows, over each of which an impressed mesial line descends obliquely outward, are a part of the anterior boundary of the brain.

From the middle of the outer convex border of the oval remains of these cups for the cerebral hemispheres, a strong blunt ridge descends obliquely down the sides of the bone to terminate the compressed anterior end of the bone just in front of the hypapophysial ridge of the sphenoid. Above this ridge the bone is much compressed anteriorly, forming a strong straight mesial keel above, which rapidly approximates to the base; the height of the bone in front being one inch and a half, which is also its extreme length.

The region below the oblique ridge is a concavity, but it is a little compressed from side to side behind, and has the same anterior compression, so that the elongated oval of the fracture at the anterior end of the bone is only three-eighths of an inch wide.

The superior ridge will probably have supported the frontals, and the anterior end would terminate in the orbito-sphenoid.

The lateral ridges appear to correspond with what Prof. Huxley has described in the Ostrich as the ridge indicative of a supra-presphenoid ossification pointed out by KÖlliker. The groove which is here noticed on the cerebral surface may indicate the same division. If so, the upper and anterior part of the mass would be the ethmoid.

This mass offers a considerable resemblance to the frontal portion of the skull of a dolphin (e. g. Delphinus delphis) from which the maxillary, premaxillary, palatine and nasal bones have been removed. But in the Porpoise the mesial ridge dividing the cerebral hemispheres is not prolonged so far forward as in the Pterodactyle; the cranial bones are often as smooth on the inside. Notwithstanding Wagner's assurance that the Pterodactyle skull is very like a Monitor's, he would have looked in vain for an ossification in Monitor, Iguana, or other Lizards, comparable with this mass. And although the brain is closed in front by bones in Serpents, it is by the frontal bones, which form a covering for nearly the whole of the conical cerebrum. Nor in the Crocodile is there any ossified mass in front of the brain, although the brain approximates nearer to Birds than is the case with other living Reptiles. Among Birds such a structure as that of the Pterodactyle is characteristic, but no bird has it so massive and mammal-like, though an approximation is made in some thick-skulled birds like Ciconia marabou. And in birds it usually is prolonged much further forward than appears to have been the case with Pterodactyle, where from the rapid tapering of the mass in front it appears to have ended in a vertical ridge like that in Parrots and Birds with a moveable beak. In Birds there is usually a median ridge dividing the cerebral hemispheres, but there is also often a small olfactory lobe prolonged in front of the cerebrum, to which nothing analogous is indicated in these fossils.